The present invention relates to a method of dressing an abrasive cloth used in the polishing of a flat surface, and an apparatus therefor. In particular, it relates to a method of dressing an abrasive cloth by using a disc-shaped or narrow annular dresser, and an apparatus therefor.
A demand has recently arisen for a particularly high level of accuracy in the polishing of a flat surface, in fields such as the polishing of semiconductor wafers. When a flat surface is created on a workpiece by a polishing device that is used for such extremely accurate polishing, it is necessary that the flatness of a surface plate that is orientated parallel to the work surface of the workpiece should be extremely high, in order to increase the accuracy of the flat surface of the workpiece.
With this type of polishing, the surface plate itself is subjected to the polishing action of an abrasive material, so that the flatness of the surface plate gradually deteriorates. To maintain this flatness of the surface plate, it is therefore necessary to recondition it by using a suitable grindstone.
While the importance of correcting the flatness of a surface plate has long been recognized, the reconditioning of such surface plates is currently performed by empirical methods. For example, to counteract any deterioration in shape of a surface plate used in lapping, which is one type of polishing, a corrective ring into which the workpiece is inserted is placed on the surface plate. The state of wear of the surface plate causes the corrective ring to move as appropriate in the radial direction of the surface plate, thus correcting the flatness of the surface plate while the lapping is being performed (refer to pages 89-90 of Precision Finishing and Special Processing, published by Meigensha in 1987).
There are many different types of polishing machines. One type polishes a workpiece against an abrasive cloth that is spread over the surface plate. The term, an abrasive cloth in the specification and claims includes a polishing pad used in the field of polishing process for semiconductor wafers. This abrasive cloth is made of an elastomer, a soft plastic, a woven material with fibers arranged in a regulated manner, or a nonwoven fabrics. With this type of polishing machine, the abrasive cloth must be dressed in a suitable manner. An abrasive cloth that has been dressed unevenly will reduce the accuracy of the flat surface of the workpiece. However, the conditions for dressing this type of abrasive cloth are still being discovered by repeated trial and error, so that there is currently no established technique therefor.